In recent years, there has been a great deal of interest in the transmission of video signals via optical fibers. This mode of signal transmission offers a number of advantages over transmitting signals by conventional coaxial cable video signal distribution as is now commonly accomplished in CATV systems. Optical fibers intrinsically have more information-carrying capacity than do coaxial cables. In addition, there is less signal attenuation in optical fibers than in coaxial cable adapted for carrying radio frequency (RF) signals. Consequently, optical fibers can span longer distances between signal regenerators than is possible with coaxial cable. In addition, the dielectric nature of optical fiber eliminates any problems with electrical shorting. Finally, optical fiber is immune to ambient electromagnetic interference (EMI) and generates no EMI of its own.
Amplitude modulation, or more specifically, intensity modulation, of an optical signal by a wideband radio frequency signal requires a light modulating device, such as a laser, which has linear characteristics over a wide dynamic range of operation. Until recently it has been difficult to fabricate lasers in which the relationship between input current and optical output was linear over more than an extremely limited range. Because of this difficulty in obtaining lasers which were sufficiently linear to support analog intensity modulation, digital intensity modulation was, until recently, the primary means for transmitting information by optical signals. However, recent advances in laser technology have made analog intensity modulation of optical signals feasible. Currently available Fabry-Perot (FP) and Distributed Feedback (DFB) lasers have sufficiently linear characteristics to allow them to be used as analog modulators of optical signals.
An important advantage of AM fiber systems for CATV is that the same multichannel NTSC, PAL, or SECAM signal format is maintained throughout the system. No format conversion electronics are required at either end of the optical link. This makes the AM fiber optic system "friendly" to the CATV system tie-in points. Because of this advantage, AM fiber optic systems generally require less equipment space in the installation. An AM system is also less costly to install, particularly on a per channel basis, than either FM or digital systems.
The single mode optical fiber used in AM fiber systems possesses attenuation characteristics which change extremely little with temperature variations, unlike coaxial cable. In most current AM fiber architectures, little compensation for the optical fiber response is required. However, the carrier-to-noise ratio (CNR) and intermodulation distortion performance (composite triple beat, composite second order) of AM fiber systems is tied directly to the relative level of the multichannel carriers which modulate the laser. Because of this, the issue of signal level control is important throughout initial equipment set-up, ongoing operation, and system maintenance.
The signals modulating an AM laser have certain ideal requirements. The laser used in the transmitter exhibits optimum performance for a given application when operated at a specific composite modulation index. The RF drive level per channel modulating the laser is the determining factor in the composite modulation index of the laser. Ideally, the modulation index of the laser should be precisely maintained at its optimum value to ensure specified system carrier-to-noise ratio and intermodulation distortion performance. If the laser modulation index is too large, the CNR performance improves, but the distortion performance is compromised. On the other hand, if the laser modulation index is too small, the distortion performance improves, but the CNR performance is compromised.
In general, a larger composite modulation index is required to meet higher system CNR specifications. However, a maximum modulation index exists for each laser. Above this index, laser distortion performance begins to deteriorate rapidly due to signal clipping. In high CNR performance systems, the laser is generally operating at or near its maximum composite modulation index. Channel loading also has an effect on laser modulation index. As channel loading increases, the laser composite modulation index increases, and the intermodulation distortion performance degrades.
The laser transmitter is also affected by variations in the headend output RF level due to other factors. The addition or removal of a coupler, tap, or other equipment in the headend wiring scheme can cause changes in the resultant headend RF output level. The headend RF output level also varies slightly with time, temperature, regular maintanance, and adjustments.
These variations in the signals modulating the laser degrade the quality of the transmitted signal, reducing the ability to delivery high quality signals such as video signals in fiber optic communications systems.
At the receiver, the quality of the received optical signal is affected by the fiber plant and the optical transmitter. The number and quality of connectors and splices used in field installation may differ from the originally specified plan, resulting in a different optical loss. If an OTDR measurement used to determine optical is inaccurate, then again the optical power will differ from that expected. The average intensity of the received optical signal may change due to maintanance or repair of the fiber plant. Re-routing an optical path will also affect the optical link loss. As discussed above, several aspects of the transmitter design and RF signal source can also affect the optical signal. The composite modulation index may change as a result of the addition or deletion of channels, variations of signal level, or other changes to the laser drive signal. Additionally, the laser diode output power may vary due to aging or temperature variation.
Receiver performance as measured by carrier-to-noise ratio (CNR) and distortions, composite triple beat (CTB) and composite second order (CSO) is generally degraded by non-ideal optical signals. As the optical input power or modulation index increases, the CNR performance of the receiver generally increases, but the contribution of the receiver to the system distortion increases. Conversely, as the optical input power or modulation index decreases, the contribution of the receiver to system distortion decreases, but the CNR performance of the receiver also decreases.
If variations in optical loss occur, the optoelectronic receiver performance may be affected. If the optical loss is greater than expected, the received optical power is lower than expected. Lower than expected received optical power results in a reduced RF output from the photodetector and optoelectronic receiver. Consequently, the input level to the receiver post-amplifier is lower. This condition increases the significance of the noise contribution of the receiver post-amplifier to the system CNR. The final result may be a degradation in system CNR. If the optical loss is less than expected, the received optical power is higher than expected. This results in an increased RF output level from the photodetector and optoelectronic receiver and generally improves the system CNR. However, with the optoelectronic receiver operating at a higher output level, its contribution to the system distortion is greater. The post-amplifier is also operating at a higher level and may contribute further to the system distortion. Consequently, there may be a degradation in system distortion performance.
Again, these factors limit the ability to deliver high quality signals such as video signal in a fiber optic communications systems.